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 Post subject: Re: spring rates
PostPosted: March 14, 2010, 2:47 pm 
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I can't imagine dual springs where the shorter one didn't go into coil bind. There's only about 1" of travel on them.

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 Post subject: Re: spring rates
PostPosted: June 14, 2010, 11:30 pm 
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SportsCarDesigner wrote:
Cheapracer implied, I think, that there would be a 30 lb jump in spring rate (or none?) when the 170 lb spring goes solid. Wrong (please don't piss on me!). It jumps from 92 lbs to 200 lbs suddenly... not gradually. That's probably not good. :cry:


I forgot about this thread.

Not at all if the springs are selected properly (and it doesn't have to be overly pedantic) then a nice meld can be had so that towards the end of the stroke a nice progressive rate can be acheived without any shock to the tyre that may cause an instantanious loss of grip (similar to bottoming out).

How any formula you can use to arrive at 2 springs stacked having less rate than either of the 2 is beyond me. Hmm - I just figured it as I wrote that, you or I have completely failed to take into account the 2 springs are now half length (same stack length) which effectively turns my 170 and 200 pounders into 340 and 400 respectively as they are now cut in half. My bad I guess.

With other factors in a car such as inertia and mass transfer, lighter springs do act at the start of travel accelerating earlier than their larger brother.

And I did take this picture weeks ago as I said I would.


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 Post subject: Re: spring rates
PostPosted: June 14, 2010, 11:41 pm 
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carguy123 wrote:
I can't imagine dual springs where the shorter one didn't go into coil bind. There's only about 1" of travel on them.


Thats usually for a different purpose, more a ride height issue, those are usually longer travel shocks and that short spring is to stop the main spring popping off it's seats during droop. If the main spring was longer the ride height would be too high, after droop (pothole, kerb) the car settles back to ride height quicker etc. Some systems use internal droop springs also.


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 Post subject: Re: spring rates
PostPosted: June 15, 2010, 11:53 am 
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cheapracer wrote:
How any formula you can use to arrive at 2 springs stacked having less rate than either of the 2 is beyond me. Hmm - I just figured it as I wrote that, you or I have completely failed to take into account the 2 springs are now half length (same stack length) which effectively turns my 170 and 200 pounders into 340 and 400 respectively as they are now cut in half. My bad I guess.

If you buy a 170 lb/in spring from a spring manufacturer, you get a 170 lb/in spring. Length is unrelated to length in terms of specifying a spring for purchase...Unless you literally mean chopping the spring in half ghetto style when you get it home, then yes you would then be cutting the number of coils in half thus doubling the rate.

A spring simply transfers a load from one side of the spring to the other. Period. If you push on one side with 100lbs force, the other end must push back with the same 100lbs force. Thus multiple springs in series can ONLY be LOWER in rate than either individual spring rate, because they are EACH being compressed by an equal 100lbs force and are NOT splitting the 100lbs force between them. The simplified equation for this is:

Equivalent Rate = 1 / (1/A + 1/B)
Where A and B are your individual spring rates

So due to the way springs work when placed in series, as mentioned above and previously by SCD, even with your "revised" rates (340/400) you now have also only doubled your combined effective spring rate to 184 lb/in...Until the springs start to bind causing the system to go non-linear, shooting up towards your non-bound spring rate, and thus throwing everything you thought you knew about the predictability of your suspension rates out the window.

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 Post subject: Re: spring rates
PostPosted: June 25, 2010, 1:22 am 
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Hi,Heres various spring rates at application.IEA car with 1600lb over the rear requires 200lb coilover mounted in the upright postion or requires a 266lb coil if its laid over 30deg. A car with 1600lb over the front needs a 400lb front shock in the upright postion so if its laid over 30 degs you'll need 532lb shock.

Spring rate front Total weight on front wheels
250 under 1200lbs
350 1200-1500
450 1600-2000

Rear rate Total on back wheels
110 1100-1200lbs
150 1200-1500lbs
Spring rate correction factor is
10deg .96
15deg .93
20deg .88
25deg .82
30deg .75
35deg .66
EG.200lb =Spring mounted straight
266lb= Spring at30deg
Hope this helps.Cheers


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 Post subject: Re:
PostPosted: December 15, 2010, 10:06 pm 
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KB58 wrote:
SkinnyG is right about that rule-of-thumb, that "in general" sports cars have wheel-rates equal to 1/2 that corner's unsprung weight. Race cars tend to have wheel-rates equal to that corner's unsprung weight.


Did you mean to say unsprung weight here? I think you meant sprung weight but I might be wrong, I'm wrong a lot.


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 Post subject: Re: spring rates
PostPosted: December 15, 2010, 10:35 pm 
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Correct... dang it, that post is still wrong? I thought I fixed it. Grrr....

"Unsprung", "sprung", pretty much the same thing... not.

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 Post subject: Re: spring rates
PostPosted: December 15, 2010, 10:47 pm 
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I was trying really hard to think "outside the box" and find out how 1/2 of unsprung weight spring-rates would be a good idea. Thanks for the clarification :D


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 Post subject: Re: spring rates
PostPosted: January 24, 2011, 2:28 am 
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SportsCarDesigner wrote:
The following is verbatim from Hyperco, right up the road from me... whose "Hypercoils have been utilized by every Indy 500 winner since 1965"

Combined Spring Rate

This is used when two springs are stacked on top of each other.

Combined Spring Rate =
(Spring Rate 'A' x Spring rate 'B')
÷ (Spring Rate 'A' + Spring Rate 'B')


For example if the rate for spring 'A' is 200 and the rate for spring 'B' is 500, the combined rate is calculated as follows:


Combined Spring Rate =
(200x500) ÷ (200+500) = 143




This doesn't add up to me. If I put a 200lb spring on top of a 200lb spring, then this calculation tells me that I will effectively have a 100lb spring. Uh, no. It's essentially the same as having one 200lb spring..

Am I missing something obvious?


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 Post subject: Re: spring rates
PostPosted: January 24, 2011, 6:28 pm 
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OK, I've thought about this some more, and it does make sense. The answer is that if you stack two identical 200lb springs on top of one another, then you have the same coil diameter, the same wire thickness, but twice the length - so it's half the spring rate.

Carry on.


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 Post subject: Re: spring rates
PostPosted: January 27, 2011, 8:39 pm 
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So there is a correction factor based on installation angle just like with shocks right?

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 Post subject: Re: spring rates
PostPosted: January 27, 2011, 10:53 pm 
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Yes, though most builders ignore that since the shocks are adjustable.

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 Post subject: Re: spring rates
PostPosted: September 12, 2011, 2:46 am 
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The problem as I see it, is the suspension layout and motion ratios need to be decided at the initial chassis and suspension linkage design and planning stage.
But most of us have no idea within a fairly wide margin, what the final all up sprung weight of the car is going to end up being. So how can you possibly do any spring design ?

You can do it, even if you have absolutely no idea what the final weight is going to be, but it requires a few logical steps and a few design decisions along the way.
Here is how I am doing mine.....

First step is to decide the function of the vehicle, competitive off road track racer, to pure recreational street, covers a wide range.
So that is the first thing you need to decide. How stiffly you really want it to ride on the springs, and what suspension travel you really need to have built into it.

Next step is to establish some initial suspension ride frequencies.
These will usually end up somewhere between 1Hz (extreme comfort) to 2Hz (factory sports car) or even higher for serious racing on smooth tracks.
But somewhere around 1.5 to 2.5 Hz would probably suit most of us here for a dual purpose car.
But the whole thing is wide open to personal preference.

Once you have decided on the required suspension frequencies, here is where the magic comes in.

There is a very simple formula to work out the static spring compression (at the wheel), for any required resonant suspension bounce frequency.
And that is:
frequency in bounces per minute = 188 divided by the square root of sprung deflection in inches at the wheel.

If you jack up your car at one corner so the spring just rattles free (theoretically), then lower it onto the ground and it sinks (say) five inches at the wheel, that is your static sprung deflection. Five inches of wheel travel. It does not matter if the weight on that particular wheel is 200 Lbs or 1,000 Lbs if it sinks five inches on it's spring at that corner, the suspension bounce frequency will always be the same.

Using the above formula, bounces/minute = 188 divided by square root of 5 inches
188 / 2.236 = 84 bounces per minute = 1.40 Hz which is fairly soft by some people's standards, but should give a good ride.

Maybe you prefer it a little firmer, but let us use five inches static sprung wheel compression as an example here.

Next you design your suspension links and decide on a suitable suspension travel, maybe three inches up and three inches down from ride height ?
Or whatever you want it to be.
And you design your coil shock mountings and linkages, and angles, to give you that six inches of total design travel at the wheel.

Once you know where and how the coil shocks mount, you can work out the motion ratio at ride height, and the total coil shock travel required to get the design plus and minus three inches of wheel travel.

Let's assume the motion ratio stays at exactly two, (to keep it simple). Total shock travel will be three inches for six inches of total wheel movement, and the shock will be exactly in the middle of it's range at ride height.
Now you measure the required height of the spring on that coil shock at the mid point of it's travel, because that is where we want it to be at ride height.

Let's assume the spring measures exactly six inches long at ride height, with the final suspension design as it now stands.
We know the wheel can only actually droop three inches, because that is how we designed it, but the design static sprung compression at the wheel needs to be the full five inches to get our required 84 bounces per minute (1.4Hz) ride frequency.

So what we need to figure out next, is the free length of the spring with the suspension theoretically at five inches of droop, even though the shock will not allow it to actually travel down that far.
You can do it on a scale drawing, or measure it on the car with one end of the shock disconnected.

As we know our motion ratio is two, if the suspension linkage was completely linear, the spring would need to compress two and a half inches for the wheel to move up five inches. That is unlikely to be exactly correct if the linkage is non linear, but measure what the spring length should be at five inches of droop.

If we find the motion ratio remains at exactly two, then our spring needs to compress two and a half inches under load to get our five inches of static sprung wheel travel. As we know the spring at loaded ride height is going to be six inches, we order springs with a free length of 2.5 plus 6 inches.

Without knowing the final weight of the car, you can design for the required suspension softness or stiffness to get the exact ride you want.

When the car is finally built, some scales, and knowing the motion ratios will get you close with spring poundages. and you can try some different springs that all have a free length of 8.5 inches.
If it sits too high fit softer springs. If too low, get stiffer springs.
With the springs fitted that give the correct design ride height, you will also end up getting the exact suspension ride frequencies you originally planned for.

This way you can design absolutely everything, and know exactly how it is going to all end up, without having the faintest idea what the final vehicle weight is going to be.
The springs themselves can be the very last thing you need to adjust to get things going as originally planned.

One final note. With rising rate suspensions, or any non linearity of spring/wheel movement, all you need to know is the exact motion ratio at final ride height, and the extended spring length at zero wheel load to get the correct static sprung wheel compression travel.
What the motion ratio is between those two points does not matter.
The spring will settle to it's design length at ride height, at the ride height motion ratio.
How it got there from full droop zero wheel load, through a possibly very non linear motion ratio, makes no difference.


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 Post subject: Re: spring rates
PostPosted: October 18, 2011, 4:10 pm 
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Location: Germany
Very nice "tutorial", still I'd like to add that most race cars feature a relation of roughly 35/65 (way for the wheel to move up/down (including driver)) and usually the rear wheels travel about 50% (++/--) more than the front wheels. In most cases the spring should reach full compression at forces beyond the force of 2,5 times the usual force (car at rest).
With variable designs of the A-arms, Suspension-pickup-points, (pushrods, rockers) ... someone can archive good results even with "any" springs. Simple dare to wait for these features of the design (or make them variable) while building the car, and think about this when the car reached its full weight.

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 Post subject: Re: spring rates
PostPosted: October 18, 2011, 5:01 pm 
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Location: Melbourne, Australia.
AceOfMace wrote:
I'd like to add that most race cars feature a relation of roughly 35/65 (way for the wheel to move up/down

That is an excellent final working figure to aim for.

Physically adding a bump rubber to the shock loses us some compression travel, and as a very rough rule of thumb when starting out, 50/50 initial design up/down travel (metal to metal) might end up being close to the desired final 35/65 by the time the bump rubber is taken into account and squashed as far as it is ever likely to go.


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